Patients with unipolar, nonpsychotic depressions are studied to determine if major depressive disorder is heterogeneous by biological, psychological, and treatment response data. Research aims include determining whether some depressions are cognitively based, while others are biological based and whether the distinction between endogeneous and nonendogenous depressions can be validated. Predictors of overall response of differential response to tricyclic antidepressants (amitriptyline and imipramine) are identified. Specific hypothesis about the relationships between the dexamethasone suppression test, sleep electroencephalograms, and psychological measures are tested. Psychological measures include self-report measures of three cognitive functions (cognitions, dyfunctional attitudes, and attributional styles).